Super-tough steel and production method thereof

ABSTRACT

A super-tough steel and a production method thereof. The present invention belongs to the field of steels and preparation thereof, and particularly relates to a super-tough steel for cryogenic services, and a production method thereof. The super-tough steel comprises the following elements in percentage by weight: 0.10% to 0.15% of C, and 29.5% to 31.5% of Mn, with the balance being Fe and inevitable impurities. The production method comprises the following steps: A1, smelting with argon protection, and electroslag remelting; A2, hot rolling or hot forging; A, annealing at 900° C. to 1100° C. for 1 hour, and quenching; and A4, cold rolling, annealing of the cold-rolled plate at 700° C.-1200° C. for 1 hour, and quenching after annealing. The steel is simple in composition and does not contain precious metals; and the steel has an average grain size of less than 30 microns, and is non-magnetic with complete face-centered cubic structure. The steel has outstanding performance at cryogenic temperatures, and its cryogenic impact energy exceeds all currently known metallic materials.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of PCT Application No. PCT/CN2020/105116, filed on Jul. 28, 2020, which claims priority to Chinese Patent Application No. 201911356888.1, filed on Dec. 25, 2019, and each application is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present invention belongs to the field of steels and preparation thereof, and particularly relates to a super-tough steel for cryogenic services, and a production method thereof.

BACKGROUND ART

Working equipment for cryogenic services, for example, the plating of satellites for collecting space debris in outer space (about 4K), cooling devices (2K-4K) in the field of cryogenic superconductivity, etc., have very strict requirements on materials.

In addition, conventional applications on the ground, for example, liquefied natural gas storage tanks (liquefaction temperature −163° C.), polar ice breakers, etc., require high toughness and impact resistance of materials.

In the above cases, common metals have the problems of brittle fracture and poor impact toughness.

At present, the structural material supporting superconducting magnets is a main research direction.

In various technologies using superconductors, such as nuclear fusion power generation, particle accelerators, superconducting energy storage and the like, superconducting magnets are used for the reason that generating a strong magnetic field needs a very large current. As superconducting magnets generally generate a large electromagnetic force and are usually cooled by liquid helium to a cryogenic temperature of 2-4 k, the structural material supporting superconducting magnets needs to have a high strength and can bear the large electromagnetic force at cryogenic temperature. In addition, the influence of the structural material to the magnetic field must be reduced to the minimum.

Existing structural materials for supporting superconducting magnets may include austenitic stainless steel, high-Mn steels, aluminum alloys, titanium alloys, and fiber reinforced plastics.

Normal austenitic stainless steels are not strong or tough enough at cryogenic temperature, while nitrogen-containing low-carbon stainless steels' austenite phase is not stable enough, and when the stainless steel is deformed at cryogenic temperature, part of the austenite phase is transformed into a ferromagnetic martensite phase, which leads to the decrease in toughness.

When an austenitic stainless steel with a further increased content of nickel is used as a structural material for cryogenic services, problems such as increased cost, high thermal expansion coefficient, etc. may be caused. Compared with austenitic stainless steel, fiber reinforced plastics is superior as it is nonmagnetic and easy to process, and is low in specific gravity, thermal expansion coefficient and unit sectional strength. In addition, although titanium alloys have low specific gravity and high strength, they have the problems of low toughness at cryogenic temperature and high cost.

Aluminum alloys are widely used for cryogenic services because of their light weight, high strength and extremely low permeability, but aluminum alloys are not strong enough and involve weldability problems.

At present, there are two types of cryogenic metals, the first type is cryogenic steels represented by 9% Ni steel. 9% Ni steel is produced via a very complicated process, involving complicated heat treatments such as twice quenching+intercritical quenching+tempering (RLT), quenching+intercritical quenching+tempering (QLT), quenching+tempering (QT) and the like, so that the cost for producing it is very high, and as it contains a large amount of noble metal Ni, the alloy is very expensive. The main structure of the steel is body-centered cubic martensite, so that the steel is magnetic and has an impact energy of less than 150 J at cryogenic temperature. As the temperature decreases, both the strength and plasticity of 9% Ni steel will increase, but the impact toughness at cryogenic temperature will decrease.

The second type is high-entropy and medium-entropy alloys, and high-entropy alloys may have an impact energy of nearly 400 J at cryogenic temperature. However, as high-entropy and medium-entropy alloys contain a large amount of noble metals Co, Ni and V, the alloys are very expensive and require ultra-pure smelting, so that the smelting cost is also very high. What's more, currently, the use of high-entropy alloys is mostly in laboratories without large-scale production. High-Mn steels are also widely used for cryogenic services, for example, as described in JP63259022A, JPH0215151A, JPS6227557A, JPS58107477A, JPS61143563A, JPH02205631A, U.S. Pat. No. 6,761,780B2, etc. Almost all the high-Mn steels disclosed in the aforementioned documents contain elements such as cadmium, nickel, niobium, and erbium.

Current researches on cryogenic impact toughness are as follows:

1. High-entropy alloys

[1] Xia, S. Q., Gao, M. C., Zhang, Y. (2017). Abnormal temperature dependence of impact toughness in AlxCoCrFeNi system high entropy alloys. Materials Chemistry and Physics, S0254058417304637.

A CoCrFeNi high-entropy alloy was disclosed, which had an impact energy of up to 397.87 J at 77 K (higher than any other metals known at present), and had an inverse temperature effect, i.e. the lower the temperature, the higher the impact value.

It was concluded in the paper that: the ability to form nano-twins and fine dimple fractures are the key to improve impact toughness.

[2] Li, D. Y, & Zhang, Y. (2016). The ultrahigh charpy impact toughness of forged AlxCoCrFeNi high entropy alloys at room and cryogenic temperatures. Intermetallics, 70, 24-28.

What was disclosed are as follows:

Al0.1CoCrFeNi: 289 J/77 K, 420 J/RT, σy: 412 MPa/77 K, σy: 250 MPa/298 K

Al0.3CoCrFeNi: 328 J/77 K, 413 J/RT, σy: 515 MPa/77 K, σy: 220 MPa/298 K

For the two materials above, both the strength and plasticity were increased as the temperature decreased, but the materials did not show an inverse temperature effect for the impact, i.e. the impact energy was reduced as the temperature decreased.

[3] Bernd Gludovatz, Anton Hohenwarter, Dhiraj Catoor, Edwin H. Chang, Easo P. George, Robert O. Ritchie. A fracture-resistant high-entropy alloy for cryogenic applications. SCIENCE. 2014, v 345: 1153-1158.

A CrMnFeCoNi alloy was disclosed, and it had a reverse temperature effect in tensile performance, i.e. both the strength and plasticity were increased when the temperature decreased, planar dislocation glide was activated at room temperature, and when the temperature decreased, twinning deformation was activated, resulting in stable work hardening ability. However, the alloy did not show a reverse temperature effect in impact toughness, and the numbers were relatively stable at cryogenic temperature:

Jk: 293 K-250 kJ/m2, 200 K-260 kJ/m 2, 77 K-255 kJ/m 2;

KJIC: 293 K-217 MPa·m1/2, 200 K-221 MPa·m1/2, 77 K-219 MPa·m1/2.

2. TWIP steel

Generally, TWIP steels have a high content of Mn (12-30%) and contain small amounts of C (<1%), Si (<3%) or Al (<3%). Its structure at room temperature was single austenite with a few annealed twins.

[4] Seok Su Sohn, Seokmin Hong, Junghoon Lee, Byeong-Chan Suh, Sung-Kyu Kim, Byeong-Joo Lee, Nack J. Kim, Sunghak Lee. Effects of Mn and Al contents on cryogenic-temperature tensile and Charpy impact properties in four austenitic high-Mn steels. 2015 Acta Materialia, 100, 39-52.

Four kinds of steels, Fe-19Mn, Fe-19Mn-2Al, Fe-22Mn and Fe-22Mn-2Al, were disclosed in this paper. Their yield strengths were greatly increased at cryogenic temperature, but the plasticities were not increased synchronously. Besides, the steels did not show a reverse temperature effect in impact toughness, and their impact toughnesses were very low.

Charpy impact energy (J)

Fe-19Mn: 83. 4±1.6 (RT) 10.3±0.2 (−196° C.)

Fe-19Mn-2Al: 87. 6±3.2 (RT) 16.8±0.9 (−196° C.)

Fe-22Mn: 84.2±1.6 (RT) 36.6±0.4 (−196° C.)

Fe-22Mn-2Al: 90. 7±1.1 (RT) 42.0±0.2 (−196° C.)

[5] Kim, H., Ha, Y, Kwon, K. H., Kang, M., Kim, N. J., & Lee, S. (2015). Interpretation of cryogenic-temperature charpy impact toughness by microstructural evolution of dynamically compressed specimens in austenitic 0.4C-(22-26)Mn steels. Acta Materialia, 87, 332-343.

This paper disclosed: when the yield strength was increased at cryogenic temperature, the plasticity was not obviously increased, and the steels did not have a reverse temperature effect for the impact at cryogenic temperature, i.e. the impact energy was reduced at cryogenic temperature (−196° C.).

At cryogenic temperature, the stacking fault energy was reduced by above 30%, as compared with that at room temperature. It was believed that the impact energy of 0.4C-22Mn was higher than that of 0.4C-24Mn and 0.4C-26Mn due to the generation of a large amount of E-martensite, leading to a TRIP mechanism while TWIP was working.

[6] Yu, L., Yufei, L., Wei, L., Mahmoud, K., Huibin, L., & Xuejun, J. (2018). Hierarchical microstructure design of a bimodal grained twinning-induced plasticity steel with excellent cryogenic mechanical properties. Acta Materialia, 158, 79-94.

This paper disclosed: both the strength and ductility of Fe-0.45C-24Mn-0.05Si-2Al-0.1Nb steel were increased as the temperature decreased, but the impact toughness at cryogenic temperature was reduced as the temperature decreased, and there was no reverse temperature effect.

The current relevant materials are summarized in the following table:

TABLE 1 Summary of Materials Trend Impact Yield Yield Impact of Yield Ductility Toughness Strength Strength Ductility Ductility Impact Toughness Strength (RT → (RT → RT −196° C. RT −196° Toughness −196° C. (RT → −196° −196° Cited Type Composition (MPa) (MPa) (%) C. RT (J) (J) −196° C.) C.) C.) Documents High- CoCrFeNi 287.44 397.87 ↑ Materials entropy (Max) chemistry alloy and physics (2018) A10.1CoCrFeNi 250 412 420 289 ↑ ↑ ↓ Inter- metallics (2016) A10.3CoCrFeNi 220 515 413 328 ↑ ↑ ↓ Inter- (slightly) metallics (2016) CrMnFeCoNi ≈400  759 ↑ ↑ — Science (2014) TWIP Fe—19Mn 438 ± 39  680 ± 51 83.4 ± 1.6 10.3 ± 0.2 ↑ ↓ ↓ Acta steel Fe—19Mn—2Al 446 ± 27 759 ± 8 84.2 ± 3.2 36.6 ± 0.4 ↑ ↓ ↓ Materialia Fe—22Mn 435 ± 44 646 ± 6 87.6 ± 3.2 16.8 ± 0.9 ↑ ↓ ↓ (2015) Fe—22Mn—2Al 442 ± 65  811 ± 14 90.7 ± 1.1 42.0 ± 0.2 ↑ ↓ ↓ TWIP Fe—0.4C—22Mn 334 528 146 62 ↑ ↓ ↓ Acta steel Fe—0.4C—24Mn 314 574 118 57 ↑ ↓ ↓ Materialia Fe—0.4C—26Mn 322 598 83 40 ↑ ↑ ↓ (2015) TWIP Fe—0.45C—24Mn- 386 ± 22  720 ± 29 49.4 ± 73.1 ± 193 ± 6  140 ± 6  ↑ ↑ ↓ Acta steel 0.05Si—2Al—0.1Nb 0.9  1.5  Materialia (As-received) (2018) Fe—0.45C—24Mn- 744 ± 27 1080 ± 22 36.6 ± 59.4 ± 198 ± 7  100 ± 6  ↑ ↑ ↓ Acta 0.05Si—2Al—0.1Nb 1.2  1.8  Materialia (BG-TWIP) (2018) TWIP Fe—17Mn—0.003C 120 50 ↓ steel Fe—25Mn—0.002C 180 35 ↓ Fe—36Mn—0.003C 270 120 ↓ 9% Ni 9% Ni 343 794 23.9   37.3  163 11 K. H. Kwonetal./ Scripta Materialia 69 (2013) 420-423 9% Ni 581 ± 21  945 ± 17 12 ±   22.5 ± 298 ± 5  283 ± 9  ↑ ↑ ↓ Acta 2.7  2.1  Materialia (2018) QLT-9% Ni 640 910 36    34    293 227 ↑ ↓ ↓ MSEA (2013) SUS316 280 350 80 ↓ Acta LHN Materialia (1998) TWIP Fe—0.4C—22Mn 334 528 146 62 ↑ ↓ ↓ Acta steel Fe—0.4C—24Mn 314 574 118 57 ↑ ↓ ↓ Materialia Fe—0.4C—26Mn 322 598 83 40 ↑ ↑ ↓ (2015) TWIP Fe—19Mn 438 680 83.4 10.3 ↑ ↓ ↓ Acta steel Fe—19Mn—2Al 446 759 84.2 36.6 ↑ ↓ ↓ Materialia Fe—22Mn 435 646 87.6 16.8 ↑ ↓ ↓ (2015) Fe—22Mn—2Al 442 811 90.7 42.0 ↑ ↓ ↓

SUMMARY OF THE INVENTION

An object of the present invention is to solve the problems of brittle fracture and low impact toughness of metallic materials at cryogenic temperatures, and to provide a metallic material which is super tough at cryogenic temperatures, having an impact energy of more than 420 J.

In the present application, the cryogenic temperature refers to liquid nitrogen temperature (−196° C.).

In order to achieve the object, the present invention provides a super-tough steel comprising the following elements in percentage by weight: 0.10% to 0.15% of C, and 29.5% to 31.5% of Mn, with the balance being Fe and inevitable impurities.

In addition, the preset invention further provides a production method of the super-tough steel, comprising the following steps:

A1, smelting, in percentage by weight, 0.10% to 0.15% of C, and 29.5% to 31.5% of Mn, with the balance being Fe and inevitable impurities in the presence of argon, and subjecting the obtained system to electroslag remelting;

A2, hot rolling or hot forging: heating ingots to a temperature of 1150° C. to 1250° C., and hot rolling or hot forging the ingots with an initial rolling temperature or initial forging temperature of ≥800° C. and a final rolling temperature or final forging temperature of ≥600° C., followed by air cooling, to obtain a hot rolled plate or hot forged plate of 20 to 40 mm;

A3, annealing the hot rolled plate or hot forged plate under a temperature of 900° C. to 1100° C. for 1 hour, and quenching the plate; and

A4, cold rolling the quenched steel plate with a cold rolling reduction of 50%-75% to obtain a cold rolled plate with a thickness of about 10 mm; and annealing the cold-rolled plate under a temperature of 700° C. to 1200° C. for 1 hour, and quenching the annealed plate.

Definitions of Elements

C: carbon is an interstitial solute element, and as the strength of steel can be effectively improved by solid-solution strengthening, the content of carbon must be controlled to be 0.05% or more in order to obtain a desired yield stress at cryogenic temperature. On the other hand, when the content of carbon exceeds 0.18%, the austenite phase is unstable, and hard-phase carbides are prone to precipitate during annealing. As a result, the magnetic permeability can no longer be maintained to be low at cryogenic temperature, leading to decrease in weldability and workability. Therefore, the content of carbon is preferably within a range of 0.10% to 0.15%.

Mn: manganese helps to stabilize the austenite phase at cryogenic temperature to achieve a very low permeability. Therefore, the content of manganese must be above 29.0%. On the other hand, if the content of manganese is excessive, the toughness, weldability, and workability will be decreased. Therefore, the content of manganese is preferably within a range of 29.5% to 31.5%.

The balance are iron and inevitable impurities. Obviously, for the inevitable impurities, the lower the content, the better. However, considering the industrial economical efficiency, it is acceptable to have Si≤0.01, S≤0.008, P≤0.008.

If the content of Si is excessive, the cryogenic impact toughness may be decreased, therefore, the content of Si is preferably to be controlled with an upper limit of 0.01% by weight.

The presence of S and P may impair the hot machinability of steel and causes cracks during welding. Accordingly, in the present invention, preferably: S is controlled below 0.008% by weight, and P is controlled below 0.008% by weight.

Explanation for the steps of the production method:

in step A1, in order to prevent volatilization of Mn during smelting, argon is used as protective gas. Thereafter, electroslag remelting is performed after the smelting is finished.

In step A2, the effects of hot forging and hot rolling are substantially the same in the present invention, as long as the temperature requirements are met.

In step A4, cold rolling may result in a fine lamellae structure, and the cold-rolled specimen is annealed to obtain a fine grain structure. A low annealing temperature may lead to a small grain size, however, when the annealing temperature is lower than 700° C., part of the specimen may be recrystallized in microstructure, which forms a hard phase and is negative to the impact performance at cryogenic temperature. Therefore, in the present invention, the annealing temperature after cold rolling is controlled within the range of 700° C. to 1200° C.

The deformation mechanism of Fe—Mn—C austenite steel is directly related with the temperature and stacking fault energy. When the stacking fault energy is lower than 18 mJ/m², ε-martensite transformation is prone to occur, and when the stacking fault energy is 12-35 mJ/m2, twinning deformation occurs mainly. The stacking fault energy in the ideal state can be calculated according to formula (1-1):

Γ=2ρΔG ^(γ→ϵ)+2σ  (1-1)

Where Γ is the stacking fault energy; ρ is the molar surface density along the {111} plane; ΔG^(γ→ε) is the molar Gibbs free energy for γ→ε; and σ is the interface energy of γ/ε.

In formula (1-1), ρ can be represented by formula (1-2):

$\begin{matrix} {\rho = {\frac{4}{\sqrt{3}}\frac{1}{a^{2}N}}} & \text{(1-2)} \end{matrix}$

Where a—lattice constant; and

N—Avogadro's constant.

For Fe—Mn—C ternary alloys, the molar Gibbs free energy ΔGγ→ε for γ→ε in formula (1-1) can be calculated according to formula (1-3):

ΔG=χ _(Fe) ΔG _(Fe) ^(γ→ε)+χ_(Mn) ΔG _(Mn) ^(γ→ε)+χ_(C) ΔG _(C) ^(γ→ε)+χ_(Fe)χ_(Mn)ΔΩ_(FeMn) ^(γ→ε)+χ_(Fe)χ_(C)ΔΩ_(FeC) ^(γ→ε)+χ_(Mn)χ_(C)ΔΩ_(MnC) ^(γ→ε) +ΔG _(mg) ^(γ→ε)   (1-3)

Where χ_(i) is the mole percentage of the ith element; ΔG_(i) ^(γ→ε) is the molar free energy required for the ith element when γ→ε transformation occurs; ΔΩ_(ij) ^(γ→ε) is the molar free energy required for the interaction between the ith element and the jth element during γ→ε transformation; and ΔG_(mg) ^(γ→ε is the stacking fault energy which is related to the grain size, and the formula for expressing the stacking fault energy with an additional excess term of grain size is shown as formula ()1-4):

Γ=2ρΔG ^(γ→ε)+2σ^(γ/ε)+2ρΔG _(ex)

  (1-4)

Where ΔG_(ex) is

ΔG _(ex)=170.06 exp(−d/18.55)  (1-5)

The above studies show that martensitic transformation is prone to occur as the temperature decreases and the stacking fault energy decreases, while grain refinement can effectively offset the decrease of the stacking fault energy caused by temperature reduction so as to partially inhibit the martensitic transformation. On the other hand, increasing the content of Mn is also positive to the inhibition of martensitic transformation. Therefore, the present invention proposes a method for inhibiting martensitic transformation by controlling the content of Mn at about 30% in combination with grain refinement.

The present invention has the following beneficial effects: by adopting the technical scheme provided by the invention, a steel which is simple in composition and does not contain precious metals can be produced; and through alloying design and structure control, a steel with stable austenite structure and an average grain size of less than 30 microns can be obtained, and is is non-magnetic with complete face-centered cubic structure. At room temperature, the yield strength in tension is greater than or equal to 220 MPa with a maximum of 230 MPa, the tensile strength can reach 520 MPa with a maximum of 531 MPa, the uniform ductility is up to 50%, the total ductility is up to 65%, and the impact energy is higher than 310 J with a maximum of 340 J; and at cryogenic temperature, the yield strength in tension is up to 360 MPa, the tensile strength is greater than 750 MPa with a maximum of 860 MPa, the uniform ductility may be about 80%, the total ductility is greater than 84%, and the impact energy is higher than 400 J with a maximum of 458 J. The steel has outstanding performance at cryogenic temperature, and its cryogenic impact energy exceeds all currently known metallic materials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a fine-grained Fe-30Mn-0.11C steel having an average grain size of 5.6 microns;

FIG. 2 is a histogram showing grain size distribution of a fine-grained Fe-30Mn-0.11C steel having an average grain size of 5.6 microns;

FIG. 3 shows stress-strain curves of Fe-30Mn-0.11C steels with different grain sizes in tension at room temperature (RT) and liquid nitrogen temperature (LNT);

FIG. 4 shows an EBSD microstructure analysis for a Fe-30Mn-0.11C steel having an average grain size of 5.6 microns after a tensile fracture happens at room temperature;

FIG. 5 shows an EBSD microstructure analysis for a Fe-30Mn-0.11C steel having an average grain size of 5.6 microns after a tensile fracture happens at liquid nitrogen temperature;

FIG. 6 shows an EBSD phase analysis for a Fe-30Mn-0.11C steel with an average grain size of 5.6 microns after a tensile fracture happens at liquid nitrogen temperature;

FIG. 7 shows a 3D reconstructed picture of a liquid nitrogen impact fracture surface of a Fe-30Mn-0.11C steel having an average grain size of 5.6 microns;

FIG. 8 shows an SEM picture of a liquid nitrogen impact fracture surface of a Fe-30 Mn-0.11C steel having an average grain size of 5.6 microns;

FIG. 9 shows a cryogenic impact energy comparison between Fe-30Mn-0.11C steels having an average grain size of 5.6 microns and different contents of Mn;

FIG. 10 shows a comparison between a Fe-30Mn-0.11C steel having an average grain size of 5.6 microns and different cryogenic metallic materials; and

FIG. 11 shows pictures of Fe-30Mn-0.11C steel specimens having an average grain size of 5.6 microns and an average grain size of 47.0 microns after impacts at liquid nitrogen temperature and room temperature.

In FIG. 11, the fine-grained Fe-30Mn-0.11C steel having an average grain size of 5.6 microns was obtained by 50% cold rolling+700° C. annealing for 1 hour; and the Fe-30Mn-0.11C steel having an average grain size of 47.0 microns was obtained without cold rolling.

DETAILED DESCRIPTION OF THE INVENTION

Comparative Example 1: 0. 05% of C, and 30.4% of Mn, with the balance being Fe and inevitable impurities, wherein Si≤0.01, S≤0.008, P≤0.008. Hot rolling was performed after smelting.

After 50% cold rolling and then annealing at 650° C. for 2 hours, a Fe-30.4% Mn-0.05% C steel having an average grain size of 1.3 microns was obtained, and the steel, in tension at room temperature, had a yield strength of up to 430 MPa, a tensile strength of 699 MPa, a uniform ductility of 40.3%, a total ductility of up to 51.7%, and an average impact energy of 278 J; and the steel had an average impact energy of 172 J at cryogenic temperature.

Comparative Example 2: 0. 05% of C, and 30.4% of Mn, with the balance being Fe and inevitable impurities, wherein Si≤0.01, S≤0.008, P≤0.008. Hot forging was performed after smelting.

After 50% cold rolling and then annealing at 1100° C. for 1 hours, a Fe-30.4% Mn-0.05% C steel having an average grain size of 20 microns was obtained, and the steel, in tension at room temperature, had a yield strength of up to 204 MPa, a tensile strength of 525 MPa, a uniform ductility of 39.4%, a total ductility of up to 51.4%, and an average impact energy of 329.6 J; and the steel had an average impact energy of 325 J at cryogenic temperature.

TABLE 2 Performance Parameters of Comparative Examples 1 and 2 Impact Energy (J) Specimens Cryogenic Average Room Temperature Temperature Grain Yield Tensile Uniform Total (Liquid Size Strength Strength Ductility Ductility Room Nitrogen (μm) (MPa) (MPa) (%) (%) Temperature Temperature) Comparative 1.3 430 699 40.3 51.7 278 172 Example 1 Comparative 20 204 525 39.4 51.4 329.6 325 Example 2

In Comparative Example 1, the annealing with lower temperature after cold rolling results in smaller average grain size, and all the indexes at room temperature were superior to that of

Comparative Example 2, except the impact energy, which was inferior to that of Comparative Example 2. In both the above comparative examples, the impact energy at cryogenic temperature was lower than that at room temperature, and the reverse temperature effect did not occur.

The following examples illustrate the properties of steels of different compositions defined herein.

Example 1: Fe-29.5% Mn-0.10% C, obtained by 50% cold rolling and then annealing at 700° C. for 1 hour.

Example 2: Fe-30% Mn-0.11% C, obtained by 50% cold rolling and then annealing at 700° C. for 1 hour.

Example 3: Fe-31.5% Mn-0.15% C, obtained by 50% cold rolling and then annealing at 700° C. for 1 hour.

TABLE 3 Impact Energies of Steel Specimens of Different Compositions at Room Temperature and Liquid Nitrogen Temperature Average Impact Impact Average Energy (J) Energy (J) Impact Impact (Liquid (Liquid Energy (J) Energy (J) Nitrogen Nitrogen (Room (Room Specimens Temperature) Temperature) Temperature) Temperature) Example 1 430 426 330 330 Fe-29.5% 425 328 Mn-0.10% C 423 332 Example 2 458 453 335 334 Fe-30% 448 329 Mn-0.11% C 452 340 Example 3 450 449 342 343 Fe-31.5% 452 340 Mn-0.15% C 445 345

In the above examples, three specimens were made in each example. It can be seen that, compared with the comparative examples, the impact energy at cryogenic temperature was greatly improved, and the reverse temperature effect did occur. Among others, the performance indexes of Example 2 were optimal.

The following examples and comparative examples illustrate the properties of Fe-30% Mn-0.11% C steels produced via different processes.

Example 2: 0.11% of C, and 30% of Mn, with the balance being Fe and inevitable impurities. The

raw materials were subjected to hot forging, annealing, and then 50% cold rolling with a rolling speed of 4.2 m/s, the deformation per pass being 1 mm/pass; after cold rolling, the cold-rolled material was annealed at 700° C. for 1 hours, and a Fe-30% Mn-0.11% C steel having an average grain size of 5.6 microns was obtained, and the steel, in tension at room temperature, had a yield strength of up to 230 MPa, a tensile strength of 531 MPa, a uniform ductility of 50%, a total ductility of up to 65%, and an average impact energy of 334 J; and the steel, in tension at cryogenic temperature, had a yield strength of up to 360 MPa, a tensile strength of 860 MPa, a uniform ductility of 70%, a total ductility of up to 84%, and an average impact energy of 453 J.

FIGS. 1 and 2 are a metallographic image and a grain size distribution histogram of the steel obtained in Example 2, and it can be seen that its average grain size is 5.6 microns.

Comparative Example 3: after hot rolling, the cold-rolled material was annealed at 1000° C. for 1 hours, and a Fe-30% Mn-0.11% C steel having an average grain size of 47 microns was obtained, and the steel, in tension at room temperature, had a yield strength of up to 180 MPa, a tensile strength of 495 MPa, a uniform ductility of 53%, a total ductility of up to 66%, and an average impact energy of 372 J; and the steel, in tension at cryogenic temperature, had a yield strength of up to 320 MPa, a tensile strength of 732 MPa, a uniform ductility of 80%, a total ductility of up to 87%, and an average impact energy of 269 J.

TABLE 4 Tensile Properties of Fe—30Mn—0.11C Steel Specimens Produced via Different Processes at Room Temperature and Liquid Nitrogen Temperature Room Temperature Liquid Nitrogen Temperature Average Yield Tensile Uniform Total Yield Tensile Uniform Total Grain Strength Strength Ductility Ductility Strength Strength Ductility Ductility Specimens Size (Mpa) (Mpa) (%) (%) (Mpa) (Mpa) (%) (%) Comparative 47.0 180 495 53 66 320 732 80 87 Example 3 (μm) Example 2 5.6 230 531 50 65 360 860 70 84 (μm)

TABLE 5 Impact Energies of Fe-30Mn-0.11C Steel Specimens Produced Via Different Processes at Room Temperature and Liquid Nitrogen Temperature Average Impact Impact Average Energy (J) Energy (J) Impact Impact (Liquid (Liquid Energy (J) Energy (J) Average Nitrogen Nitrogen (Room (Room Grain Tempero- Temper- Temper- Temper- Specimens Size ature) ature) ature) ature) Comparative 47.0 265 269 378 372 Example 3 (μm) 290 372 251 365 Example 2  5.6 458 453 335 334 (μm) 448 329 452 340

In Comparative Example 3, the specimen was produced without cold rolling, thereby maintaining a relatively large grain size. It can be seen from above that, as compared with Comparative Example 3, in Example 2, the specimen that subjected to cold rolling had a smaller grain size, and its comprehensive performance, especially at liquid nitrogen temperature, was outstanding.

Stress-strain curves of Example 2 and Comparative Example 3 in tension at room temperature (RT) and liquid nitrogen temperature (LNT) are shown in FIG. 3.

Example 4: 0.11% of C, and 30% of Mn, with the balance being Fe and inevitable impurities. The

raw materials were subjected to hot forging, annealing, and then 50% cold rolling with a rolling speed of 4.2 m/s, the deformation per pass being 1 mm/pass; after cold rolling, the cold-rolled material was annealed at 1200° C. for 1 hours, and a Fe-30% Mn-0.11% C steel having an average grain size of 26 microns was obtained, and the steel, in tension at room temperature, had a yield strength of up to 220 MPa, a tensile strength of 520 MPa, a uniform ductility of 54%, a total ductility of up to 65%, and an average impact energy of 315J; and the steel, in tension at cryogenic temperature, had a yield strength of up to 280 MPa, a tensile strength of 777 MPa, a uniform ductility of 82%, a total ductility of up to 86%, and an average impact energy at liquid nitrogen temperature of 423 J.

Example 5: 0.11% of C, and 30% of Mn, with the balance being Fe and inevitable impurities. The

raw materials were subjected to hot forging, annealing, and then 50% cold rolling with a rolling speed of 4.2 m/s, the deformation per pass being 1 mm/pass; after cold rolling, the cold-rolled material was annealed at 900° C. for 1 hours, and a Fe-30% Mn-0.11% C steel having an average grain size of 10.7 microns was obtained, and the steel, in tension at room temperature, had a yield strength of up to 225 MPa, a tensile strength of 524 MPa, a uniform ductility of 55%, a total ductility of up to 66%, and an average impact energy of 320J; and the steel, in tension at cryogenic temperature, had a yield strength of up to 285 MPa, a tensile strength of 818 MPa, a uniform ductility of 83%, a total ductility of up to 85%, and an average impact energy at liquid nitrogen temperature of 421 J.

TABLE 6 Tensile Properties of Fe—30Mn—0.11C Steel Specimens Produced via Different Hot Treatments after Cold Rolling at Room Temperature and Liquid Nitrogen Temperature Room Temperature Liquid Nitrogen Temperature Average Yield Tensile Uniform Total Yield Tensile Uniform Total Grain Strength Strength Ductility Ductility Strength Strength Ductility Ductility Specimens Size (Mpa) (Mpa) (%) (%) (Mpa) (Mpa) (%) (%) Example 2  5.6 (μm) 230 531 50 65 360 860 70 84 Example 4   26 (μm) 220 520 54 65 280 777 82 86 Example 5 10.7 (μm) 225 524 55 66 285 818 83 85

TABLE 7 Impact Energies of Fe-30Mn-0.11C Steel Specimens Produced via Different Hot Treatments after Cold Rolling at Room Temperature and Liquid Nitrogen Temperature Average Impact Impact Average Energy (J) Energy (J) Impact Impact (Liquid (Liquid Energy (J) Energy (J) Average Nitrogen Nitrogen (Room (Room Grain Temper- Temper- Temper- Temper- Specimens Size ature) ature) ature) ature) Example 2  5.6 (μm) 458 453 335 334 448 329 452 340 Example 4  26 (μm) 408 423 317 315 430 309 432 319 Example 5 10.7 (μm) 423 421 328 320 433 325 407 317

The above examples only differ in the annealing temperature after cold rolling, while other process conditions are substantially the same. A low annealing temperature may result in a small average grain size, while a high annealing temperature may result in a large average grain size. The steel with a small average grain size has the highest impact energy at liquid nitrogen temperature.

The figures show the properties of a fine-grained Fe-30Mn-0.11C steel having an average grain size of 5.6 microns, which has the best cryogenic performance.

As can be seen from FIG. 4, after being stretched at room temperature, the material mainly had dislocation deformations in the microstructure of parallel ends formed by a tensile fracture.

As can be seen from FIG. 5, after being stretched at liquid nitrogen temperature, the material mainly had dislocation deformations and twinning deformations.

As can be seen from FIG. 6, after a fracture happened at liquid nitrogen temperature, the material had no martensitic transformation.

As can be seen from FIG. 7, there is a large shrinkage near the impact fracture surface, and besides the main crack, there were secondary cracks in other directions. However, even if the impact was applied at liquid nitrogen temperature, the specimen was not broken into two pieces.

As can be seen from FIG. 8, the impact fracture surface had a dimple-like feature of submicron size.

As can be seen from FIG. 9, the steel disclosed in the present invention is super-tough at cryogenic temperature and has an impact energy of greater than 450 J, as compared with other high manganese steels.

As can be seen from FIG. 10, the steel disclosed in the present invention having an average grain size of 5.6 microns is super-tough at cryogenic temperature and has an impact energy of more than 450 J at liquid nitrogen temperature. In addition, compared with other metals, the steel also has a comparable yield strength at room temperature.

In FIG. 11:

a is a picture of a Fe-30Mn-0.11C steel specimen having a grain size of 5.6 microns after receiving an impact at liquid nitrogen temperature;

b is a picture of a Fe-30Mn-0.11C steel specimen having a grain size of 5.6 microns after receiving an impact at room temperature;

c is a picture of a Fe-30Mn-0.11C steel specimen having a grain size of 47.0 microns after receiving an impact at liquid nitrogen temperature; and

d is a picture of a Fe-30Mn-0.11C steel specimen having a grain size of 47.0 microns after receiving an impact at room temperature.

It can be seen that both the specimens are not broken by impact at either room temperature or liquid nitrogen temperature.

According to the invention, the steel is endowed with stable austenite structure via alloying design, and is endowed with an average grain size of smaller than 30 microns in microstructure via structure control, and the steel is non-magnetic with complete face-centered cubic structure. The fine-grained structure of this composition, when stretched or impacted at room temperature or cryogenic temperature, does not generate brittle phase martensite at cryogenic temperature, and when deformed, generates a lot of compatible twinning deformations. The impact fracture surface has massive fine dimples of submicron size, and there is a large shrinkage near the impact fracture surface, thereby absorbing a lot of impact energy.

According to the present invention, the prepared steel plate has an impact energy at cryogenic temperature higher than that of all currently known metal materials, having a wide prospect in cryogenic applications. 

1. A super-tough steel, characterized by comprising the following elements in percentage by weight: 0.10% to 0.15% of C, and 29.5% to 31.5% of Mn, with the balance being Fe and inevitable impurities.
 2. The steel according to claim 1, characterized in that the steel comprises, in percentage by weight, 0.11% of C, and 30% of Mn, with the balance being Fe and inevitable impurities.
 3. The steel according to claim 1, characterized in that the steel has an average grain size of less than 30 microns in microstructure.
 4. The steel according to claim 3, characterized in that the steel has an average grain size of 5.6 microns in microstructure.
 5. The steel according to claim 1, characterized in that the steel has an average impact energy of greater than 420 J at cryogenic temperature (liquid nitrogen temperature).
 6. A production method for the super-tough steel, comprising the following steps: A1, smelting, in percentage by weight, 0.10% to 0.15% of C, and 29.5% to 31.5% of Mn, with the balance being Fe and inevitable impurities in the presence of argon, and subjecting the obtained system to electroslag remelting; A2, hot rolling or hot forging: heating ingots to a temperature of 1150° C. to 1250° C., and hot rolling or hot forging the ingots with an initial rolling temperature or initial forging temperature of ≥800° C. and a final rolling temperature or final forging temperature of ≥600° C., followed by air cooling, to obtain a hot rolled plate or hot forged plate of 20 to 40 mm; A3, annealing the hot rolled plate or hot forged plate at a temperature of 900° C. to 1100° C. for 1 hour, and quenching the plate; and A4, cold rolling the quenched steel plate with a cold rolling reduction of 50%-75% to obtain a cold rolled plate with a thickness of about 10 mm; and annealing the cold-rolled plate under a temperature of 700° C. to 1200° C. for 1 hour, and quenching the annealed plate. 